TESTACEA. 55 



had enlarged to at least six times their original dimensions ; and on the 

 21st, some young had escaped from the third, thus requiring eighteen 

 days to attain maturity. 



When the spawn is origiuall\' deposited, as represented figs. 18, 19, 

 natural size and enlarged, a considerable time elapses until it begins to 

 swell. The embryos are now further dispersed, and more removed from 

 each other, whereby they become much more distinct to view. The 

 capsule is opening to discharge the albuminous substance whereon they 

 are imbedded 



Specimens taken on the 11th of August, afterwards spawned on the 

 side of their glass in globules of the purest albumen, rendered greenish 

 by the embryonic specks, each of which was contained in its own integu- 

 ment. These were considerably dispersed on the 24th by the swelhng 

 of the spawn, the capsule opening to discharge it, fig. 22. A portion en- 

 larged appeared as fig. 23. 



At this time the motion of the embryos in their peculiar integu- 

 ments had commenced, and some were so far advanced, that pulsation 

 of the heart was quite distinct. In two days more, one of the globules, 

 originally half a line in diameter, extended three lines, still extremely 

 transparent, with the embryos widely dispersed. Improving strength 

 and organization enabled the embryo to pursue a course around the in- 

 terior of its prison, fig. 24, a, b. But the shape was clumsy and less ac- 

 curately proportioned than in the adult, thougli some of the organs were 

 sufficiently evident. 



Now the heart is seen as a vessel of absolute transparency, with dis- 

 tinct pulsation, as discovered from the sides or integument. No other 

 internal organization can be clearly recognised, and this only when the 

 subject is in certain positions. No cilia have been detected with magni- 

 fiers of considerable power. 



Should the young perish soon after exclusion, their shells exhibits 

 only a single complete volution, universally indented by circles, as if the 

 whole were composed of circular rings, fig. 24, a, b. 



The process of evolution above described is singular, and demands 



